Trivalent chromium conversion coating and method of application thereof

ABSTRACT

The present invention is directed to a high protection, trivalent chromium coating composition that is particularly useful as a conversion coating on substrates in need of corrosion protection. The conversion coating composition comprises chromium (III) ions, cobalt (II) ions, nitrate ions, and sulfate ions. The invention further provides methods of applying a conversion coating to a plated article and articles with a conversion coating applied thereto.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/684,939, filed May 26, 2005, which is incorporated herein in itsentirety.

FIELD OF THE INVENTION

The present invention is generally related to the treatment of a metalsurface for improving the properties thereof, particularly the corrosionresistance of the metal surface. More specifically, the invention isrelated to conversion coatings, preferably coatings useful for applyingto a metal plated surface.

BACKGROUND

Conversion coatings are commonly used in the metal finishing industry toprovide improved properties to metal surfaces, particularly metal platedsurfaces. Conversion coatings are generally known to be particularlyuseful for providing improved corrosion resistance to the metal and alsofor improving adhesion properties of the metal in relation to additionalcoatings that may later be added, such as paint or other finishes.Multiple types of conversion coatings are known and used in theindustry, and one type that is particularly known to be beneficial ischromating.

Chromate conversion coatings can be applied at various thicknesses andcan provide varying levels of corrosion protection, as well as otherproperties. For example, such coatings can range from a very thin“blue-bright” finish to a very thick “olive-drab” finish. Blue-brightfinishes are transparent with a slight blue tint and high luster. Such afinish not only imparts a corrosion-resistant coating to the surface ofthe substrate but also aesthetically enhances the substrate and articlesmade therefrom. Heavier chromate conversion coatings are considerablymore protective than the bright finishes, but they do not meet theaesthetic criteria that are characteristic of the bright coatings. Theseheavier coatings are well-recognized by their yellow, bronze, orolive-drab finishes, which correspond in general order to increasingfilm thickness.

While chromate conversion coatings are particularly useful for providingcorrosion resistance, such coatings historically have been prepared fromhexavalent chromate (i.e., chromium (VI) ions). This makes such chromateconversion coatings unfavorable since chromium (VI) is generallyrecognized as being toxic. The National Institute for OccupationalSafety and Health (NIOSH), for example, considers all chromium (VI)compounds to be potential occupational carcinogens. The Centers forDisease Control (CDC) and NIOSH report an increased risk of lung cancerhas been demonstrated in workers exposed to chromium (VI) compounds.Other adverse health effects associated with chromium (VI) exposureinclude dermal irritation, skin ulceration, allergic contact dermatitis,occupational asthma, nasal irritation and ulceration, perforated nasalsepta, rhinitis, nose bleed, respiratory irritation, nasal cancer, sinuscancer, eye irritation and damage, perforated eardrums, kidney damage,liver damage, pulmonary congestion and edema, epigastric pain, anderosion and discoloration of the teeth. Given these findings, the wastefrom a hexavalent chromium based solution creates significantenvironmental concerns, and hexavalent chromium baths require specialtreatment prior to disposal.

Other types of treatments for forming passivation coatings that arenon-chromium containing are known in the art, but these are generallyunsatisfactory for enhancing the corrosion resistance of a platedsubstrate, particularly when the treated substrate is subjected to ahumid environment. Such treatments typically include phosphatetreatments and a bright dipping step, followed by a coating step using atransparent lacquer. Each step provides an additional barrier layer tocorrosive conditions, but not a cohesive film forming a chemical bondbetween a film-forming element and the coated substrate as is achievedby hexavalent chromium processes. Accordingly, phosphate treatmentsprovide barriers that are porous, permitting moisture to pass through tothe coated substrate. Phosphate coatings are also not bright inappearance, but provide dull, opaque, paint-like overcoats lacking theaesthetic attributes of hexavalent chromium conversion coatings.

Treatments that are free of hexavalent chromium have been developedutilizing trivalent chromium (i.e., chromium (III) ions). For example,multiple U.S. Patents describe solutions or processes for treating metalsurfaces, wherein the solution comprises chromium ions, substantiallyall of which are in the trivalent state (see, generally, U.S. Pat. No.4,349,392; U.S. Pat. No. 4,359,345; U.S. Pat. No. 4,359,346; U.S. Pat.No. 4,359,348; U.S. Pat. No. 4,367,099; U.S. Pat. No. 4,384,902; U.S.Pat. No. 4,578,122; and U.S. Pat. No. 6,096,140). In each of the citedU.S. patents, an oxidizing agent is included as an essential ingredientin the hexavalent chromium-free solution. It is known in the art thatthe presence of oxidizing agents may lead to some conversion oftrivalent chromium to hexavalent chromium during the formation of theconversion coating. The presence of this hexavalent chromium may thenlead to some or all of the previously described health and handlingproblems. Furthermore, the inclusion of an oxidizing agent may addadditional costs and steps to the formation of the conversion coatings.

Accordingly, it would be useful to have a hexavalent chromium-freeconversion coating that provides similar attributes of traditionalconversion coatings that are based on hexavalent chromium. Furthermore,it is desirable that such a hexavalent chromium-free coating issubstantially impervious to moisture and is able to provide excellentcorrosion resistance for an extended period of exposure. Preferentially,the coating prepared without hexavalent chromium should alsosimultaneously enhance the appearance of the substrate by imparting anattractive, brightly polished finish.

SUMMARY OF THE INVENTION

The present invention provides a high protection, trivalent chromiumcoating composition that is particularly useful as a conversion coatingon substrates in need of corrosion protection. The invention alsoprovides methods for applying a conversion coating to such substrates,and coated articles prepared according to the method. The trivalentchromium coating composition is particularly useful in that it is freeof hexavalent chromium and is also free of components that mayfacilitate conversion of trivalent chromium to hexavalent chromium.Further, the trivalent chromium coating composition is beneficial inthat it provides improved corrosion protection, particularly extendingthe time to which a substrate having a conversion coating according tothe invention can be exposed to a corrosive environment withoutsubstrate corrosion occurring.

In one aspect of the invention, there is provided a conversion coatingcomposition. In one embodiment, the composition comprises chromium (III)ions, cobalt (II) ions, nitrate ions, and sulfate ions. Preferably, thechromium (III) ions are present at a concentration of at least about 0.1moles/L, and the nitrate ions are present at a concentration of at leastabout 0.4 moles/L. In one particular embodiment, the conversion coatingcomposition comprises at least about 0.01 moles/L of cobalt (II) ions,and at least about 0.1 moles/L of sulfate ions.

Preferably, the conversion coating composition of the invention is freeof components that may reduce the corrosion resistance properties of theconversion coating formed of the inventive composition. For example, inone embodiment, the conversion coating composition particularly avoidsthe use of components that may provide free fluorides. In anotherembodiment of the invention, the conversion coating is substantiallyfree of chelators, in particular, carboxylic acid based chelators (suchas di-oic acids).

The conversion coating composition of the invention can further compriseone or more additional components useful for imparting desirableproperties to an article with a conversion coating applied thereto.Non-limiting examples of additional components that may be useful in thecompositions of the invention include aminocarboxylic acids or salts orderivatives thereof, silicates, such as clays (particularlynanoparticulate clays), and halogenated derivatives of acetic acid, orsalts or esters thereof.

The conversion coating composition of the invention is particularlyuseful in that it can be prepared as a concentrate. The concentrateprovides ease of storage and transportation, and it can easily bediluted at the time of use to provide the conversion coating compositionof the invention.

According to another aspect of the invention, there is provided a methodfor applying a conversion coating to an article having an exposedsurface. In one embodiment, the method comprises contacting the exposedsurface of the article with a conversion coating composition comprisingchromium (III) ions, cobalt (II) ions, nitrate ions, and sulfate ions.In one particular embodiment, the chromium ions are present at aconcentration of at least 0.1 moles/L and the nitrate ions are presentat a concentration of at least about 0.5 moles/L.

In one particular embodiment according to this aspect of the invention,the exposed surface of the article has been plated with a metal, such aszinc or a zinc alloy. Accordingly, the method of the invention cancomprise plating an article and contacting the plated surface of thearticle with a conversion coating composition according to theinvention. The method can comprise further steps, such as surfacepreparation steps prior to the plating step, or rinsing and drying ofthe article after contacting it with the conversion coating composition.

The invention also encompasses articles having a conversion coatingapplied thereto. In one embodiment, the invention provides an articlehaving an exposed surface with a conversion coating applied thereto,wherein the conversion coating is applied according to the method of theinvention. Such coated articles are particularly beneficial for theanti-corrosion protection provided to the article by the appliedconversion coating. In one embodiment, the article with the conversioncoating applied thereto exhibits anti-corrosion protection arising fromthe conversion coating such that the article with the conversion coatingapplied thereto can withstand a salt spray according to ASTM testingmethod B 117-03 for a time of at least about 200 hours before formationof white salts corrosion products.

DETAILED DESCRIPTION OF THE INVENTION

The present inventions now will be described more fully hereinafter withreference to specific embodiments of the invention. Indeed, theinvention may be embodied in many different forms and should not beconstrued as limited to the embodiments set forth herein; rather, theseembodiments are provided so that this disclosure will satisfy applicablelegal requirements. As used in the specification, and in the appendedclaims, the singular forms “a”, “an”, “the”, include plural referentsunless the context clearly dictates otherwise.

In various embodiments, the invention may be described herein in termsof being “substantially free” of certain compounds, elements, ions, orother like components. Accordingly, as used in describing the invention,“substantially free” is intended to mean that the compound, element,ion, or other like component is present, at most, in only trace amounts(i.e., a concentration so minute that the presence of the compound,element, ion, or other like component will have no adverse affect on thedesired properties of the coating). Preferably, “substantially free”indicates the specified compound, element, ion, or other like componentis completely absent or is not present in any amount measurable bytechniques generally used in the art.

The conversion coating composition of the invention is substantiallyfree of chromium (VI) ions. Accordingly, the composition contains, atmost, only trace amounts of chromium (VI) ions in such a minuteconcentration that the presence of the chromium (VI) ions will have noaffect on the desired properties of the coating and will have no adversehealth or environmental impact. Most preferably, the inventivecomposition contains no chromium (VI) ions.

The conversion coating composition of the invention is alsosubstantially free of oxidizing agents. Such oxidizing agents aregenerally understood to include agents, such as peroxides andpersulfates, that are known to effect conversion of trivalent chromiumto hexavalent chromium. Most preferably, the inventive compositioncontains no oxidizing agents capable of converting trivalent chromium tohexavalent chromium.

The conversion coating composition of the invention generally compriseschromium (III) ions, cobalt (II) ions, and nitrate ions. In furtherembodiments, the conversion coating composition also comprises sulfateions. The mixture of the various types of ions, particularly inspecified concentrations, is useful for forming a conversion coating onan article having an exposed surface, thereby providing improvedcorrosion resistance for the article.

According to certain embodiments of the invention, the chromium (III)ions present in the conversion coating composition can be characterizedas being present in at least a minimum concentration. For example, inone embodiment, a chromium (III) source is provided in an amount suchthat the conversion coating composition includes chromium (III) ions ata concentration of at least about 0.1 moles/L. Preferably, thecomposition includes chromium (III) ions at a concentration of at leastabout 0.15 moles/L, or at least about 0.2 moles/L. The chromium (III)ions may also be characterized, according to the invention, as beingpresent in a preferable concentration range. For example, in oneembodiment, the composition includes chromium (III) ions at aconcentration of about 0.1 moles/L to about 0.4 moles/L. In furtherembodiments, the chromium (III) ions are present at a concentration ofabout 0.15 moles/L to about 0.35 moles/L, about 0.15 to about 0.30moles/L, or about 0.15 to about 0.25 moles/L. In one particularembodiment, the chromium (III) ions are present at a concentration ofabout 0.20 moles/L.

A cobalt (II) source is preferably provided in an amount such that theconversion coating composition comprises cobalt (II) ions at aconcentration of at least about 0.01 moles/L. Preferably, thecomposition comprises cobalt (II) ions at a concentration of at leastabout 0.02 moles/L, at least about 0.03 moles/L, or at least about 0.04moles/L. Typically, the conversion coating composition comprises about0.01 moles/L to about 0.10 moles/L of cobalt (II) ions. In furtherembodiment, the conversion coating composition comprises about 0.02moles/L to about 0.09 moles/L, about 0.03 moles/L to about 0.08 moles/L,or about 0.04 moles/L to about 0.08 moles/L of cobalt (II) ions.

The chromium (III) source and the cobalt (II) source can both be anysource capable of providing free chromium (III) ions and cobalt (II)ions in solution. For example, in one embodiment, the chromium (III) ionsource and the cobalt (II) ion source are chosen from various salts ofthe metals. According to such an embodiment, it is understood that thesource can also introduce ions other than chromium (III) ions or cobalt(II) ions. It is preferred that the chromium (III) source and the cobalt(II) source not include components that could be detrimental to thecorrosion resistance properties of the conversion coating composition.

In certain embodiments of the invention, it is beneficial for theconversion coating to comprise further ionic components that, insolution, enhance or improve the corrosion resistance ability of thecomposition. One example of such further beneficial ions is sulfate (SO₄⁻²) ions. Without being bound by theory, it is believed that sulfateions function as film formers on zinc plated surfaces. Sulfate ions canalso act as a buffer, facilitating control of solution pH while alsoenhancing the stability of the solution. Beneficially, such desirablefurther ionic components can be added to the inventive compositionwithout the need for additional ingredients, which could otherwiseincrease the cost of preparing the composition or be detrimental to theeffectiveness of the composition. Accordingly, it is useful for thechromium (III) source and the cobalt (II) source to comprise compoundsthat also include the further desirable ions. For example, in oneembodiment, the chromium (III) source and the cobalt (II) source can beCr₂(SO₄)₃ and CoSO₄, respectively.

The conversion coating composition of the invention can comprise sulfateions at a concentration of at least about 0.1 moles/L. Preferably,sulfate ions are present at a concentration of at least about 0.2moles/L or about at least 0.3 moles/L. In further embodiments, sulfateions are present at a concentration of about 0.2 moles/L to about 1.0moles/L. Moreover, sulfate ions may be present in the composition at aconcentration of about 0.25 moles/L to about 0.90 moles/L, 0.30 moles/Lto about 0.80 moles/L, or about 0.35 moles/L to about 0.80 moles/L.

The chromium (III) ions and cobalt (II) ions can also be providedthrough other sources, such as other metal salts. While sulfate salts ofchromium (III) and cobalt (II) are particularly useful, as describedabove, any chromium (III) salt or cobalt (II) salt could be usedprovided the salt does not contribute components that could bedetrimental to the anti-corrosive properties of the conversion coatingcomposition. Non-limiting examples of other metal salts that could beused in the invention include inorganic salts, such as nitrate salts orchloride salts, and organic salts, such as acetate salts.

Preferably, according to one embodiment, a source of nitrate ions isprovided such that the composition comprises at least about 0.4 moles/Lof nitrate ions. In further embodiments, the conversion coatingcomposition comprises at least about 0.5 moles/L of nitrate ions or atleast about 0.6 moles/L of nitrate ions. Typically, the conversioncoating composition comprises nitrate ions at a concentration of about0.4 moles/L to about 1.5 moles/L. In yet further embodiments, theconversion coating composition comprises about 0.5 moles/L to about 1.3moles/L of nitrate ions, or about 0.6 moles/L to about 1.2 moles/L.

Any source capable of providing a sufficient concentration of nitrateions without introducing additional components that could be detrimentalto the ability of the composition to impart corrosion resistance to anarticle can be used according to the invention. Further, such a sourceshould not be or form an oxidizing agent capable of converting trivalentchromium into hexavalent chromium. In one embodiment, NaNO₃ is used toprovide nitrate ions to the composition. In a further embodiment, nitricacid may be used as a source of nitrate ions for the conversion coatingcomposition. Preferably, if nitric acid is added to the solution, theamount of nitric acid added is below a level resulting in oxidation oftrivalent chromium to hexavalent chromium. Other non-limiting examplesof nitrate sources that could be used according to the invention includepotassium nitrate, chromous nitrate, cobalt nitrate, and ammoniumnitrate.

In light of the concentrations provided above for the chromium (III)ions, the cobalt (II) ions, and the nitrate ions, the conversion coatingcomposition of the invention can be particularly defined in terms of themole ratio of nitrate ions to the combination of chromium (III) ions andcobalt (II) ions [NO₃ ⁻/(Cr⁺³+Co⁺²)]. In one embodiment, the mole ratioof nitrate ions to combined chromate and cobalt ions is greater thanabout 1.5:1. Preferably, the ratio is at least about 1.75:1, morepreferably at least about 2:1, and most preferably at least about 2.5:1.In a further embodiment, the mole ratio of nitrate ions to combinedchromium and cobalt ions is at least about 3:1, at least about 4:1, atleast about 5:1, at least about 6:1, or at least about 7:1.

The conversion coating composition of the invention can include furtheringredients generally recognizable by one of skill in the art as beinguseful for enhancing or improving corrosion resistance or for beingbeneficial in a conversion coating composition in particular. Forexample, specific compounds known as useful for inhibiting corrosion canbe included in the composition. Non-limiting examples of such corrosioninhibitors include aminocarboxylic acids and salts or derivativesthereof. One specific example of such compounds5-(dimethylamino)-2,2-diphenylpentanoic acid, which is commerciallyavailable under the tradename HALOX® 510 from HALOX, a division ofHammond Group, Inc., Hammond, Ind.

Further non-limiting examples of additional components useful in theconversion coating composition of the invention include silicates. Asused herein, silicates are understood to refer to compounds containingsilicon, oxygen, and one or more metals with or without hydrogen,including silicate esters formed by the combination of silicon andoxygen with one or more organic groups. Moreover, the silicates caninclude compounds wherein the oxygen is partially or completely replacedwith one or more different atoms, such as fluorine (e.g.,hexafluorosilicates). Silicates can be useful for increasing corrosionprotection when applied to an article as part of the conversion coatingcomposition. While not intending to limit the scope of silicates thatmay be used according to the invention, in particular, silicates usefulin the conversion coating include talc, mica, and clays, such asnanoparticulate clays. For example, the composition can include one ormore LAPONITE® clays, available from Rockwood Additives, Ltd.Specifically, the composition may include LAPONITE® RD (comprised ofSiO₂, MgO, LiO₂, and Na₂O) or LAPONITE® RDS (comprised of SiO₂, MgO,LiO₂, Na₂O, and P₂O₅). Another example of silicates useful in theinvention includes magnesium hexafluorosilicate (MgSiF₆.6H₂O).

Still further example of silicates useful according to the presentinvention are natural or synthetic phyllosilicate clays. Illustrative ofsuch materials are smectic clay minerals such as montmorillonite,nontronite, beidellite, bentonite, volkonskoite, laponite, hectorite,saponite, sauconite, magadite, kenyaite, stevensite and the like, aswell as vermiculite, halloysite, aluminate oxides, hydrotalcite and thelike. These layered clays generally comprise particles containing aplurality of silicate platelets having a thickness of about 8-12 Åtightly bound together at interlayer spacings of 4 Å or less, andcontain exchangeable cations such as Na⁺, Ca⁺², K⁺ or Mg⁺² present atthe interlayer surfaces.

In further embodiments, the conversion coating of the invention may alsocomprise one or more halogenated carboxylic acids, or salts or estersthereof, provided such carboxylic acids do not also substantiallyfunction as chelators. For example, the composition may comprisehalogenated acetic acid compounds, or salts or esters thereof, such astrifluoro acetic acid (CF₃CO₂H), sodium trifluororoacetate (CF₃CO₂Na),trichloro acetic acid (CCl₃CO₂H), sodium trichloroacetate (CCl₃CO₂Na),and the like.

While the preferred components of the conversion coating composition aregenerally described above, the coating composition of the invention canalso be characterized in that it is preferably substantially free ofcertain specific compounds, elements, ions, or the like. In oneembodiment, the conversion coating composition of the invention issubstantially free of fluoride ions. Preferably, the conversion coatingcomposition is totally free of fluoride ions. This preference is notintended to limit the use of fluorine-containing compounds generally,such as trifluoro acetic acid. Rather, the composition should notinclude free fluoride ions.

The conversion coating composition of the invention can include other,non-fluoride containing, halogen components; however, such halogencomponents are not specifically required according to the invention. Forexample, in one embodiment, the conversion coating composition caninclude chloride ions at a concentration up to about 1 moles/L. Inanother embodiment, chloride ions are present at a concentration up toabout 0.75 moles/L. In yet another embodiment, chloride ions are presentat a concentration up to about 0.5 moles/L.

It is also preferred that the conversion coating composition of theinvention be substantially free of chelators. A chelator is generallyrecognized in the art as a compound, often an organic compound, capableof forming two or more coordination bonds with a central metal ion.Chelators can be capable of coordinating metals in general, or may bemore specific for metal of certain valences (e.g., divalent cationchelators). According to the present invention, it is preferred that theconversion coating composition be substantially free of chelatorsgenerally. In one preferred embodiment, the conversion coatingcomposition is substantially free of chelators, in particular,carboxylic acid based chelators. Such carboxylic acid based chelatorswould be recognized as generally encompassing carboxylic acid compoundsincluding two or more carboxylic acid functional groups (e.g., “di-oicacids”).

The conversion coating composition of the invention can be prepared in aproperly diluted form that is ready to use. Alternately, the conversioncoating composition may be provided in a concentrated form that isdiluted prior to application of the composition to the article. In oneembodiment of the invention, a concentrate is provided wherein, uponproper dilution (such as with water), the diluted concentrate forms aconversion coating composition according to the invention. According toone, non-limiting example, a concentrate is provided wherein upondilution of about 25% by volume (i.e., 25 parts concentrate to 75 partsdiluent), the diluted concentrate forms a conversion coating compositioncomprising about 0.1 moles/L to about 0.4 moles/L of chromium (III)ions, about 0.01 moles/L to about 0.10 moles/L of cobalt (II) ions,about 0.4 moles/L to about 1.5 moles/L of nitrate ions, and about 0.2moles/L to about 1.0 moles/L of sulfate ions.

The conversion coating composition of the invention is particularlyuseful in a method for applying a conversion coating to an article withan exposed surface to provide the article with corrosion resistance. Inone embodiment, the method comprises contacting the exposed surface ofthe article with a conversion coating composition according to theinvention.

According to the method of the invention, the conversion coatingcomposition is particularly useful for applying a conversion coating onan article wherein the exposed surface of the article is plated with ametal coating by electrodeposition. In one particular embodiment, theexposed surface of the article is plated with zinc or a zinc alloy.Non-limiting examples of zinc alloys useful as plating the exposedsurface of the article include ZnSn alloys, ZnNi alloys, ZnFe alloys,and ZnCo alloys.

The article can be plated according to any plating method generallyrecognized in the art as being useful for plating zinc or zinc alloy.For example, the conversion coating, and method of application thereof,can be used in combination with plating techniques, such as cyanide,alkaline non-cyanide, sulfate-zinc, and chloride zinc plating methods.

The underlying article with the plating and conversion coating appliedthereto is not limited by the present invention. Rather, any articlerecognizable by one of skill in the art as being subject to plating andas benefiting from the corrosion resistance provided by the conversioncoating of the invention can be used in the method of the invention.Specific metals, such as iron-containing alloys, are particularlysusceptible to corrosion and are therefore particularly benefited by themethod of the invention in terms of corrosion resistance. Accordingly,while the method of the invention may be exemplified herein by referenceto iron-containing articles, such as steel articles, the invention isnot limited to such specific embodiments.

In an exemplary metal treatment operation, an iron-containing alloy,such as steel, is subject to multiple steps in preparing for andcarrying out application of the conversion coating. The steel article isprepared for plating by cleaning the article and, optionally, acidtreating the article. Preferably, the article is rinsed after thecleaning and optional acid treating steps. The article is thenelectroplated with zinc or a zinc alloy. After plating, the plated metalis rinsed, optionally exposed to a mild inorganic acid (such as nitricacid or sulfuric acid) to oxidize the surface, and rinsed again. Theconversion coating is applied according to the invention, and thearticle with the conversion coating applied thereto is rinsed and dried.It is understood the method of the invention can include only some ofthe above steps, in addition to the step of applying the conversioncoating.

The conversion coating composition can be applied to the plated articleaccording to any method generally recognized in the art as being usefulfor applying a solution to an article to allow for chemical bondformation. For example, the application of the conversion coatingcomposition can be by spraying, dipping, immersing, rolling, or othersimilar methods. In one particular embodiment, the article is immersedin a bath comprising the conversion coating composition of theinvention.

When a bath is used for immersing the article for application of theconversion coating, it is understood that the bath, in addition to theconversion coating composition of the invention, can further includeadditional components standard in the industry that may be necessary toformation of a bath but do not necessarily affect the conversion coatingapplied to the article. For example, it is understood that such a bathwould include water.

Specifically, a bath of the conversion coating composition ispreferentially prepared using a clean tank. The tank, or the tanklining, is preferably made from a material inert to the conversioncoating, such as polyethylene, polyvinyl chloride (PVC), stainlesssteel, or the like. In one particular embodiment, water is first addedto the tank. Then, while mixing, the appropriate amount of theconcentrated form of the conversion coating composition is added.Finally, the rest of the working volume of the tank is filled with watermaking the conversion coating composition properly diluted and ready forapplication to the article.

Application of the conversion coating to the plated article ispreferably carried out under specified conditions. For example, in oneembodiment of the invention, the pH of the bath containing theconversion coating composition is maintained within a certain range.Preferably, the pH of the conversion coating composition is acidic(i.e., less than about 7, and more preferably, less than about 4). Inspecific embodiments, the pH of the coating is about 2 to about 3, about2.2 to about 2.8, or about 2.4 to about 2.8.

According to another embodiment of the invention, the conversion coatingis applied to the plated article at a temperature of at least aboutambient temperature. In one preferred embodiment, the temperature of theconversion coating composition during application to the article iselevated above ambient temperature. Such elevated temperature isparticularly useful in that it has been found to improve the ultimatecorrosion resistance of the conversion coating applied to the articlewhen applied at the elevated temperature. In certain embodiments, thetemperature during application of the conversion coating composition isbetween about 20° C. and about 70° C., about 30° C. and about 60° C., orabout 40° C. and about 50° C.

The period of time during which the conversion coating composition isapplied to the article can vary depending upon the other methodparameters, such as the method of applying the conversion coatingcomposition to the article, the dilution of the composition, and thetemperature of the composition. In one embodiment, the conversioncoating composition is applied by immersing the article in a bathincluding the composition. According to this embodiment, the article isimmersed in the bath for a period of up to about 90 seconds.Specifically, the article may be immersed for about 15 seconds to about75 seconds, about 20 seconds to about 70 seconds, or about 30 seconds toabout 60 seconds.

In light of the conversion coating composition and the method ofapplication thereof, as described above, the invention further providesan article having an exposed surface that has a conversion coatingapplied to the exposed surface thereof. Such an article coated with theconversion coating of the invention is particularly useful in that theconversion coating applied thereto is effective for providinganti-corrosion protection that greatly exceeds the protection providedby the plating alone.

The anti-corrosion effects provided by the conversion coating of theinvention are easily evaluated by salt spray (or salt fog) testingperformed according to the standards of the American Society for Testingand Materials (ASTM) designation B 117-03 (October 2003 version). Thecompositions provided by the invention may be evaluated by other knowntesting methods and are intended to provide coatings capable of meetingor exceeding performance requirements that may be described in terms ofone or more different testing methods. For the sake of simplicity,however, the ability of the compositions of the present invention toresist corrosion is described herein in terms of the noted testingmethod. Accordingly, a material coated with a coating composition withinthe boundaries of the present invention should not be viewed as beingoutside the realm of the invention simply through evaluation with amethod different from ASTM B 117-03.

According to ASTM B 117-03, the testing apparatus consists of a fogchamber, a salt solution reservoir, a supply of a suitably conditionedcompressed air, and atomizing nozzles. Using the apparatus, a saltsolution comprised of about 5 parts by weight (pbw) NaCl in 95 pbw wateris sprayed onto specimens for continuous prolonged periods to causecorrosion. Depending upon the specimen used, time to corrosion can beevaluated.

Specifically related to the present invention, corrosion testing isperformed on zinc plated steel specimens to which the conversion coatingof the invention has been applied. The onset of two types of corrosionproducts is documented in the test: white salts and red rust. The visualappearance of white salts indicates failure of the conversion coatingand corrosion of the underlying zinc plating. The visual appearance ofred rust indicates failure of the zinc plating and corrosion of theunderlying steel specimen. For purposes of evaluating anti-corrosionprotection of the conversion coating of the present invention, the timeto formation of white salts corrosion products is used.

In one embodiment of the invention, an article having the conversioncoating of the invention applied thereto is characterized in that theconversion coating provides anti-corrosion protection such that whensubjected to a salt spray according to ASTM testing method B 117-03described above, the conversion coating is resistant to formation ofwhite salts for a time of at least about 200 hours. According furtherembodiments, the conversion coating is resistant to formation of whitesalts for a time of at least about 250 hours, at least about 300 hours,at least about 350 hours, or at least about 400 hours.

In addition to anti-corrosion protection, the conversion coating of theinvention is preferably aesthetically pleasing. For example, in oneembodiment, the conversion coating applied to an article according tothe invention is a bright-blue coating. Furthermore, a colored chromatecoating can be achieved by exposing a coated article to a dye, such asthe Mordant family of diazo dyes, without degrading the corrosionprotection of the coating. For example, a yellow coating can be producedwith the use of Mordant Orange 6 dye. A suitable dye exposure time isabout 5 seconds to about 40 seconds, whereby the dye is at a temperaturebetween about 20° C. and about 40° C.

The present invention is further advantageous in that the conversioncoating can be formed without pre-baking the article to be coated torelieve any hydrogen-embrittlement. Other products require the parts tobe baked prior to chromating, which also requires the surface of theparts to be reactivated prior to chromating. Baking usually destroyscorrosion protection. Chromating and then baking greatly simplifies theconversion coating process.

EXPERIMENTAL

The present invention is more fully illustrated by the followingexamples, which are set forth to illustrate various embodiments of theinvention and are not to be construed as limiting thereof.

Example 1

A series of test parts were produced to evaluate various chromateformulations. The parts were plated in acid-chloride zinc electroplatingconsisting of 3 oz/gal zinc metal, 18 oz/gal ammonium chloride, 4%(vol/vol) Smart Zinc Carrier (available from Pavco, Inc.), and 0.2%(vol/vol) Smart Zinc Toner (available from Pavco, Inc.). The parts werezinc plated at 20 amps/ft² for 25-30 minutes to produce a zinc thicknessof approximately 0.3 to 0.5 mm. After plating, the parts were rinsed inwater and then rinsed in 0.5% (vol/vol) nitric acid solution.

Various different conversion coating compositions were applied to theplated parts prepared as described above. Application was by dipping ina bath comprising a conversion coating composition according to theinvention at a temperature of 50° C. and a pH of 2.6 for 60 seconds. Theparts with the conversion coating composition applied thereto weretested for corrosion resistance according to ASTM B 117-03, as describedabove. Corrosion resistance was determined as a time to formation ofwhite salts. Conversion coating composition ingredients (in moles/L),mole ratio of nitrate ions to combined chromium (III) and cobalt (II)ions, and time to white salts (in hours) are provided in Table 1. TABLE1 [Cr⁺³] [Co⁺²] [NO₃ ⁻] [SO₄ ⁻²] [Cl⁻] Mole Ratio Time to white Run (M)(M) (M) (M) (M) NO₃ ⁻/(Cr⁺³ + Co⁺²) salts (hrs) 1 0.10 0.04 1.1 0.190.00 7.9 402.0 2 0.30 0.04 1.1 0.49 0.00 3.2 402.0 3 0.206 0.024 1.10.33 0.00 4.8 334.7 4 0.10 0.04 1.1 0.34 0.00 7.9 481.0 5 0.30 0.04 1.10.94 0.00 3.2 439.0 6 0.26 0.04 1.1 0.82 0.00 3.7 434.6 7 0.20 0.02 0.60.62 0.00 2.7 324.6 8 0.10 0.40 1.1 0.19 1.00 7.9 301.0 9 0.30 0.04 1.10.49 1.00 3.2 395.0 10  0.26 0.025 0.75 0.42 0.25 2.6 502.3 11  0.260.025 1.75 0.42 0.25 6.1 463.3 12  0.26 0.025 1.75 0.42 0.75 6.1 440.313  0.30 0.04 1.1 0.94 1.0 3.2 398.0 14  0.20 0.02 1.1 0.62 0.50 5.0351.4 15  0.26 0.025 1.75 0.805 0.25 6.1 392.8 16* 0.26 0.025 1.2 0.8050.25 4.2 >555*Note: Run 16 also included 2.5 g/L of HALOX ® 510[5-(dimethylamino)-2,2-diphenylpentanoic acid] corrosion inhibitor

As a comparative to the above data, a commercially available trivalentchromium based conversion coating was also tested according to the sameparameters as described above. A bath containing 14.5% (vol/vol)HyproBlue (available from Pavco, Inc.) at a pH of 2.4 and a temperatureof 40° C. was used. A plated part was immersed in the bath for 60seconds. Three separate parts were tested under salt spray according toASTM B 117-90. Time to white salts for the three parts was 135.6 hrs,169.7 hrs, and 115.7 hrs.

Example 2

Various test parts were prepared as described in Example 1 and werecoated with further conversion coating compositions according to thepresent invention to evaluate the ability of the coatings to resistcorrosion. Application was by dipping in a bath comprising theconversion coating composition a temperature of 40 or 50° C. and a pH of2.6 for a time of 60 seconds. The parts with the conversion coatingcomposition applied thereto were tested for corrosion resistanceaccording to ASTM B 117-03, as described above. Conversion coatingcomposition ingredients (in moles/L), mole ratio of nitrate ions tocombined chromium (III) and cobalt (II) ions, and time to white salts(in hours) are provided below in Table 2. TABLE 2 Mole Ratio Time to[Cr⁺³] [Co⁺²] [NO₃ ⁻] [SO₄ ⁻²] NO₃ ⁻/ white Run (M) (M) (M) (M) (Cr⁺³ +Co⁺²) salts (hrs) 17^(a) 0.260 0.027 1.200 0.660 4.2 303 18^(b) 0.2000.040 1.206 0.040 5.0 477 18^(c) 0.200 0.040 1.206 0.040 5.0 608 200.200 0.040 1.000 0.340 4.2 317 21 0.200 0.040 0.900 0.520 3.8 272 220.200 0.020 1.200 0.260 5.5 300 23 0.200 0.060 0.600 0.780 2.3 24924^(d) 0.200 0.040 0.800 0.240 3.3 417 25^(e) 0.200 0.040 0.800 0.2403.3 335 26^(f) 0.200 0.040 0.800 0.240 3.3 390^(a)Also included 7.5 g/L LAPONITE ® RD^(b)Also included 0.200 M CF₃CO₂H^(c)Also included 0.399 M CF₃CO₂H^(d)Also included 0.400 M CCl₃CO₂H^(e)Also included 0.600 M CCl₃CO₂H^(f)Also included 0.018 M MgSiF₆.6H₂O

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions. Therefore, it is to be understood that theinventions are not to be limited to the specific embodiments disclosedand that modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limitation.

1. A conversion coating composition comprising: (a) at least about 0.1moles/L of chromium (III) ions; (b) at least about 0.4 moles/L ofnitrate ions; (c) cobalt (II) ions; and (d) sulfate ions.
 2. Theconversion coating composition of claim 1, comprising at least about 0.2moles/L of chromium (III) ions.
 3. The conversion coating composition ofclaim 1, comprising at least about 0.6 moles/L of nitrate ions.
 4. Theconversion coating composition of claim 1, wherein the concentration ofcobalt (II) ions is at least about 0.01 moles/L.
 5. The conversioncoating composition of claim 4, wherein the concentration of cobalt (II)ions is at least about 0.02 moles/L.
 6. The conversion coatingcomposition of claim 1, wherein the concentration of sulfate ions is atleast about 0.1 moles/L.
 7. The conversion coating composition of claim6, wherein the concentration of sulfate ions is at least about 0.2moles/L.
 8. The conversion coating composition of claim 1, wherein saidcomposition is free of fluoride ions.
 9. The conversion coatingcomposition of claim 1, wherein said composition is substantially freeof chelators.
 10. The conversion coating composition of claim 1, whereinthe ratio of nitrate ions to the combination of chromium (III) ions andcobalt (II) ions is greater than 1.5:1.
 11. The conversion coatingcomposition of claim 1, further comprising one or more additionalcomponents selected from the group consisting of silicate-containingcompounds and halogenated carboxylic acids, or salts or esters thereof.12. The conversion coating composition of claim 11, wherein saidsilicate-containing compounds are selected from the group consisting ofclays and magnesium hexafluorosilicate.
 13. The conversion coatingcomposition of claim 12, wherein said clays comprise nanoparticulateclays.
 14. The conversion coating composition of claim 11, wherein saidhalogenated carboxylic acids are selected from the group consisting ofhalogenated acetic acids and salts or esters thereof.
 15. The conversioncoating composition of claim 14, wherein said halogenated acetic acidscomprise trifluoro acetic acid, trichloro acetic acid, or salts oresters thereof.
 16. The conversion coating composition of claim 1,further comprising up to about 1.0 moles/L of chloride ions.
 17. Theconversion coating composition of claim 1, wherein the chromium (III)ions and the cobalt (II) ions are supplied as metal salts.
 18. Theconversion coating composition of claim 17, wherein the metal saltcomprises a sulfate metal salt.
 19. The conversion coating compositionof claim 1 comprising: (a) about 0.1 moles/L to about 0.4 moles/L ofchromium (III) ions; (b) about 0.02 moles/L to about 0.09 moles/L ofcobalt (II) ions; (c) about 0.4 moles/L to about 1.5 moles/L of nitrateions; and (d) about 0.2 moles/L to about 1.0 moles/L of sulfate ions;wherein the composition is free of fluoride ions and chelators.
 20. Amethod for applying a conversion coating to an article having an exposedsurface, said method comprising contacting the exposed surface of thearticle with a conversion coating composition according to claim
 1. 21.The method of claim 20, wherein the exposed surface of the article isplated with zinc or a zinc alloy.
 22. The method of claim 20, whereinthe pH of the conversion coating composition is between about 2.0 andabout 3.0.
 23. The method of claim 20, wherein the temperature of theconversion coating composition is between about 20° C. and about 70° C.24. The method of claim 20, wherein said contacting step is carried outfor a time of up to about 90 seconds.
 25. The method of claim 20,wherein said contacting step comprises immersing the article in theconversion coating composition.
 26. An article having an exposed surfacewith a conversion coating applied thereto, wherein said conversioncoating is applied according to the method of claim
 20. 27. The articleof claim 26, wherein the exposed surface of the article is plated withzinc or a zinc alloy.
 28. The article of claim 26, wherein theconversion coating applied to the exposed surface is effective forproviding anti-corrosion protection such that the conversion coating,when subjected to a salt spray according to ASTM testing method B117-03, is resistant to formation of white salts corrosion products fora time of at least about 200 hours.
 29. The article of claim 28, whereinthe conversion coating applied to the exposed surface is resistant toformation of white salts corrosion products for a time of at least about300 hours.